1. Field of the Invention
The present invention relates to a liquid crystal display and its manufacture method, and more particularly to a liquid crystal display having aluminum wiring and its manufacture method. In this specification, the term “aluminum” is intended to cover both aluminum and aluminum alloy.
2. Description of the Related Art
A liquid crystal display has the advantages that it is light in weight, can be driven at low voltage and has a low consumption power. Liquid crystal displays are widely used nowadays as various types of displays. In general, a liquid crystal display has the structure that liquid crystal is sealed between two transparent glass substrates. A black matrix, color filters, a common electrode, an alignment film and the like are formed on the inner surface of one glass substrate, and on the inner surface of the other glass substrate, thin film transistors (TFTs), gate wirings, signal wirings, pixel electrodes, an alignment film and the like are formed.
One pixel unit is formed, for example, by disposing three primary color filters and corresponding three pixel electrodes. A number of pixel units are disposed in a matrix shape, for example, several hundred rows and one thousand and several hundred columns to constitute a flat panel color display. In operation, pixel rows are sequentially selected by gate electrode wirings, and image signals are applied from signal wirings to the pixel electrodes of the same row at the same time.
An inverse stagger type thin film transistor is formed by forming gate wirings (including gate electrodes) of Cr or the like on a glass substrate, covering the gate wirings with a gate insulating film, and thereafter forming an amorphous silicon layer to be used as a channel layer. After a channel protective layer is formed on each channel region of the amorphous silicon film, a metal layer is formed on source/drain regions on both sides of each channel region, the metal layer being a high impurity concentration amorphous silicon layer for source/drain contacts (source/drain electrodes and signal wirings). After the metal layer is patterned, an insulating protective film is formed to cover each thin film transistor.
Contact holes are formed through the insulating protective layer, and a transparent electrode layer of indium tin oxide (ITO) or the like is formed and patterned into pixel electrodes. In this manner, the pixel structure of an active matrix substrate is formed.
For a normal type thin film transistor (with gate electrode above the channel region), after island regions of an amorphous silicon layer used as a channel layer are formed, a middle region of each island silicon layer is covered with a gate insulating film and a gate electrode is formed on the gate insulating film. Ions are implanted by using the gate electrode as a mask to form high impurity concentration source/drain regions. If the gate insulating film is patterned to make it extend outside the gate electrode, lightly doped drain (LDD) regions can be formed.
An operation speed of TFT of a liquid crystal display depends largely upon resistance and stray capacitance of each gate wiring line. It is effective to lower the gate wiring resistance in order to improve the operation speed. If aluminum having a low resistivity is used instead of refractory metal such as Cr, the gate wiring resistance can be reduced greatly.
However, if aluminum is used as the material of gate wiring, the gate dielectric breakdown voltage becomes very low so that it is difficult to use TFT as a switching element. The reason for this has been ascribed to poor heat resistance of aluminum and hillocks formed during a heat treatment used for the manufacture of TFTs.
If refractory metal such as Cr and Ta is used, the gate dielectric breakdown voltage becomes high so that TFTs can be used as switching elements. However, since refractory metal has a high specific electric resistivity, a switching speed lowers.
It has been proposed to form gate wirings by using a lamination layer of an aluminum layer having a low electric resistivity and a refractory metal layer highly resistant against heat (for example, refer to Japanese Patent Laid-open Publication SHO-64-84668). If an aluminum layer is covered with a refractory metal layer such as Cr, deformation such as hillocks of the covered aluminum hardly occurs.
However, if a lamination layer of an aluminum layer and a refractory metal layer is patterned, the side walls of the aluminum layer are exposed. There is a possibility that hillocks are grown from the side walls. It has been proposed to first pattern an aluminum layer, then stack a refractory metal layer and thereafter pattern the refractory metal layer by using another mask to cover the whole surface of the aluminum layer (for example, refer to Japanese Patent Laid-open Publication HEI-6-120503).
Since two patterning processes are performed by using different masks to form gate wirings (including gate electrodes), not only the number of masks increases, but also the size precision of TFTs is degraded.
It has also been proposed that an aluminum layer and a first refractory metal layer are stacked and patterned, and thereafter a second refractory metal layer is deposited and anisotropically etched to leave side walls of the second refractory metal layer only on the side walls of patterned gate wirings (for example, refer to Japanese Patent Laid-open Publication HEI-11-87716).
Since the upper surface of the aluminum layer is covered with the first refractory metal layer and the side walls thereof are covered with the second refractory metal layer, the formation of hillocks can be prevented efficiently. The aluminum layer and first refractory metal layer can be patterned by using the same mask and the second refractory metal layer can be patterned by anisotropic etching without a mask. The number of masks will not increase and the size precision can be improved. It is however necessary to add a deposition process for the second refractory metal layer and an anisotropic etching process.
As described above, various proposals have been made to use aluminum and reduce the gate wiring resistance of thin film transistors.
If low resistance aluminum is used as the material of wirings of a liquid crystal display, there arises the problem of formation of hillocks or whiskers. In order to prevent the formation of hillocks or whiskers, additional processes are necessary.